The field of the invention relates to heat pumping engines and more particularly to acoustical heat pumping engines without moving seals.
An important task for a heat engine is the pumping of heat from one thermal reservoir at a first temperature to a second thermal reservoir at a second higher temperature by the expenditure of mechanical work. A Stirling engine is an example of a device which, when used with an ideal gas, can pump heat reversibly. Such an engine has two mechanical elements, a power piston and a displacer, the motions of which are phased with respect to one another to achieve the desired result. W. E. Gifford and R. C. Longsworth describe in an article entitled, "Pulse-Tube Refrigeration" which appeared August 1964 in the Transactions of the ASME on pp. 264-268, an intrinsically irreversible engine which they call a pulse-tube refrigerator or a surface heat pumping refrigerator which, in principle, requires only one moving element and which achieves the necessary phasing between temperature changes and fluid velocity by using the time delay for thermal contact between a primary gas medium and a second thermodynamic medium, in their case the walls of a stainless steel tube. The Gifford and Longsworth device utilizes, instead of a power piston, a rotating valve which cyclically at a rate of about 1 Hz connects their tube to high and low pressure reservoirs maintained by a compressor. Apparatus in accordance with the present invention utilizes the surface heat pumping principle but increases the frequency of operation by a factor of about one hundred over the frequency of the Gifford and Longsworth device. The present invention utilizes not a compressor, but an acoustical driver, thereby eliminating all moving seals and any need for external mechanical inertial devices such as flywheels.
One prior art device of interest is a traveling wave heat engine described in U.S. Pat. No. 4,114,380 to Ceperley. This device utilizes a compressible fluid in a tubular housing and an acoustical traveling wave. Thermal energy is added to the fluid on one side of a second thermodynamic medium and thermal energy is extracted from the fluid on the other side of the second thermodynamic medium. The material between the two sides is retained in approximate thermal equilibrium with the fluid, thereby causing a temperature gradient in the fluid to remain essentially stationary. The operation of this device is different from that of the instant invention in several respects. The device of this reference uses traveling acoustical waves for which the local oscillating pressure p is necessarily equal to the product of the acoustical impedance .rho.c and the local velocity v at every point of the engine while the instant invention uses standing acoustical waves for which the condition p&gt;&gt;.rho.cv can be achieved in the vicinity of the second thermodynamic medium, thereby enhancing the ratio of thermodynamic to viscously dissipative effects. Traveling waves require that no reflections occur in the system; such a condition is difficult to achieve because the second medium acts as an obstacle which tends to reflect the waves. Additionally, a thermodynamically efficient pure traveling wave system is more difficult to achieve technically than a standing wave system. The '380 invention also requires that the primary fluid be in excellent local thermal equilibrium with the second medium. This has the effect of making it closely analogous to the Stirling engine. However, the requirement on the fluid geometry necessary to give good thermal equilibrium together with the requirement that p=.rho.cv for a traveling wave imposes necessarily a large viscous loss (excepting fluids of exceedingly low Prandtl number that are unknown). The present invention utilizes imperfect thermal contact with the second medium as an essential element of the heat pumping process. As a consequence, an engine in accordance with the invention need not necessarily have the high viscous losses of the '380 traveling wave engine.
U.S. Pat. No. 3,237,421 to Gifford describes the surface heat pumping device discussed in the previously cited article by Gifford and Longsworth. The instant invention differs from the '421 device not only as described above but also in that the regenerator required between the pressure source and the surface heat pumping part of the '421 apparatus is not needed in the instant invention. Indeed, including such a regenerator in the instant invention would degrade its performance as a consequence of the same viscous heating problems that characterize the '380 invention. Too, Gifford requires a large and necessarily heavy compressor whereas the instant invention is light weight, requiring no such compressor. The Gifford device also requires moving seals while the instant invention does not.